The invention relates to data compression processes and particularly to a method for determining preferred quantizing factors which limit degradation in data signal quality incurred during multi-generation compression and decompression cycles given a selected cumulative complexity, to substantially the degradation incurred in a generation with the same cumulative complexity. The method also concerns supplying an optimal rounding algorithm and estimating the quantizing factors applied in a previous generation if they are not explicitly available.
Typical television broadcast and post production equipment and processes have used digital technology to take advantage of the power and flexibility inherent in the digital format. The advantages in using the digital format are particularly evident when applied to the manipulation of images in, for example, post production processes during which the information content of the images may be increased, decreased and/or rearranged such as is done in the process commonly known as "layering". In such processes, the storage and manipulation of television images using digital technology was first performed with random access disc recorders. The utility of the digital disc recorders was not so much in their random access capability, as it was in their ability to re-record images for multiple generations without the loss of fidelity caused, for example, when recording multiple generations with analog recorders.
By way of explanation of the terms "multiple generations" or "multi-generations" as employed in the description herein, each sequence of recording and playing back of a video image signal results in the reproduction of a copy that is a "generation" removed from the original or preceding video image signal. Thus, "multi-generations" defines a sequence of selected repetitions of an image signal through a given signal process such as, for example, consecutive recorded tape reproductions, post production editing or layering processes, etc. In practice, some deterioration of the video image signal occurs during the record and reproduce processes because of imperfections in the record and reproduce equipment. This deterioration is manifested in the reproduced video images as signal amplitude, frequency and/or phase errors. The extent of deterioration is cumulative and thus increases markedly as the number of record and playback sequences, that is, "generations" experienced by the video image signal increases. Even in the absence of layering, a few record/playback sequences of an analog video image through imperfect or misadjusted equipment, such as may occur during electronic editing processes, produce severe deterioration in the final generation of the video image signal obtained.
It is well known that the size and cost of any present or future digital recorder is directly related to its data rate and storage capacity. Accordingly, there are increasing numbers of applications in which there is a strong incentive to reduce these requirements as, for example, by using some form of data compression. This is particularly true for digital video tape recorders, disc recorders and other high density television and data recorders. Various configurations of these recorders commonly are used in television post production processes, where 10 to 20 generations of, for example, video image data are common and more than 50 generations are occasional requirements. To be acceptable, a compression algorithm used in these recorders must meet the same multi-generation requirements as are imposed on D-1 component and D-2 composite color video tape recording standards, that is, a 4.5 image rating on the CCIR 600 five point scale.
In the event that image data is compressed before being recorded by a digital disc or an analog or digital tape recorder, it is well known that the process experiences an inherent initial quantization error which is generally a one time error occurrence. In the absence of uncorrectable recorder errors, initial quantization errors generally do not get worse with subsequent multiple generations of compression and decompression. This is only true, however, if there is no additional processing of the television image between the generations. Image processing such as the process of image "layering" mentioned above, wherein additional image data are superimposed on, or undesired image data are removed from, an existing video image, introduces quantization roundoff errors, otherwise known as rounding errors. The rounding errors are the inherent distortion introduced into the signal by the quantization process, wherein a range of values is divided into a finite number of smaller sub-ranges, each of which is represented by an assigned or "quantized" value within the sub-range. The introduction of rounding errors in subsequent generations after the first generation, will degrade the signal-to-noise ratio in some way with each generation. Fortunately it is extremely rare for any one area of an image to be processed more than a few times even if it is re-recorded very many times.
In the case of compression and decompression cycles an additional requirement is that the compression ratio, that is, the quantizing factor, is constant for each area of the image, for each generation. In order to achieve some worthwhile compression ratio, most compression algorithms do not compress all areas of the image to the same degree. Busy areas in the picture produce more data than quiet areas and thus a coarser quantizing factor generally is applied to the data in busy areas. Accordingly, in typical data compression processes, the average compression parameters are selected and/or adjusted to provide the desired total recorded data per image. That is, in video image compression applications it generally is desirable to compress video images to a particular data block size (in bits) or average data rate (in bits per second). The quantizing factor, other parameters being constant, is chosen to yield the desired compressed data block size or average data rate that will just fit the data into a desired data block length, and/or into an allotted recording space on the disc or tape. To this end, some compression algorithms adjust their parameters dynamically from block to block of data. Others adjust a single set of parameters each generation for use over a relatively large area of the image, up to the entire image.
It has been determined that if the same image is compressed and decompressed many generations, it will not suffer any substantial degradation beyond the first generation, particularly if the same quantizing factor is used for each area. However, if the image is changed in some way between generations, that is, if an image layering process is performed, and the usual different optimum quantizing factors which precisely fit the compressed data into the allotted space are used to achieve the same compression in each generation, the complexities of the conditions can lead to a rapid degradation in image quality. In addition, the normal rounding technique of adding a constant 0.5 and truncating the fractional part, accelerates the degradation.
It follows that it would be highly desirable to provide a compression/decompression technique for accomplishing multiple generation image layering without degrading image quality, wherein each generation introduces an increased or decreased layer of data over a portion of the existing image whereby the layer is more or less complex than the portion of the image it replaces. Such multiple layering processing results in slightly more, or slightly less, compression being required with each generation. The technique should compensate for the increase or decrease in compression required, while still maintaining the optimal image fidelity throughout the successive generations after the first.